In a dot matrix printer, each character is printed as a pattern of individual elements--dots, bars, and other shapes may be used. The most common type of dot matrix print head utilizes a series of rods as print elements. The print rods are driven selectively into impacting relation with the recording medium, which may be an impact-sensitive paper or the combination of ordinary paper and an inked ribbon. Another dot matrix print head uses tiny individual resistance heaters as the print elements, in combination with a thermally-responsive recording paper.
A variety of different constructions have been proposed for dot matrix thermal print heads. Perhaps the most efficient and effective basic construction employs a plurality of very thin conductive films superimposed upon a dielectric substrate. One of the films is of a high-resistance material; tiny individual areas of that film constitute the resistance heater elements used to form the printed characters. Low-resistance high-conductivity metal films afford selective electrical circuit connections to the resistance heater print elements. Quite frequently, an outer wear protection film is used to prevent undue wear on the print head surface, which must be maintained in pressure contact with the recording paper during the printing operation. The wear protection film is preferably non-conductive.
In these thin film dot matrix thermal print heads, it is also known to provide additional films for obtaining effective adhesion between the basic operational films, particularly between the conductive film used for electrical connections and the films employed for the resistance heater elements and for wear protection. It is almost always necessary to have the conductive films superimposed upon each other in order to afford effective electrical connections between the films; the films are too thin to provide effective electrical connections in abutting relationship.
Although the films employed in the fabrication of a dot matrix thermal print head of the kind described are extremely thin, with their thicknesses being measured in Angstroms, the usual construction of a thin film thermal print head nevertheless leaves definite depressions and pockets between the electrical connectors, which are usually the thickest part of the composite film structure. Furthermore, the high-resistance film is usually located, for practical reasons, at the bottom of the superimposed film layers. This results in several distinct disadvantages which impose substantial limitations on the operating life of the print head. In particular, the electrical connector films, at the outer surface of the print head, may be subject to excessive abrasion from the necessary pressure contact with the recording paper. This can definitely shorten the life of the print head.
Another distinct problem results from the collection of carbonized thermally reactive chemicals, present in the recording paper, and burnt paper lint in the depressions and pockets between the conductors. This accumulation ultimately reaches a level that tends to separate the print head from the paper, causing a decrease in print quality. Furthermore, the accumulated deposits have many characteristics similar to ink and tend to smear the recording paper. This problem is accentuated in those constructions in which the resistive elements are located immediately upon the surface of the print head base, with a pocket for accumulation of carbonized chemicals and burnt paper lint immediately over each print element. There, accumulation creates a thermal barrier between the resistance heater print element and the paper leading to marked deterioration in print quality.